When high-frequency pulse current is applied to an amorphous wire comprising a Co—Fe—Si—B alloy, wire impedance varies depending on a magnetic field due to the skin effect. This is a known phenomenon called the magneto-impedance effect (hereinafter referred to as the “MI effect”). A high-sensitive magnetic sensor using an MI element, which detects this variation by a detection coil wound around the amorphous wire, namely, an MI sensor is now used in a cell phone or the like. However, conventional MI sensors have a problem of poor hysteresis characteristics. This problem is caused by magnetic domain structure of the amorphous wire serving as a magneto-sensitive wire.
An example of magneto-sensitive wires used in conventional MI sensors is disclosed in Domestic Re-Publication of PCT International Application No. WO2005/019851. This publication discloses a tension-annealed amorphous wire of 20 μm in diameter and an MI element of not more than 1 mm in length and 0.5 mm in height employing this wire. The amorphous wire disclosed in this publication, however, has large hysteresis characteristics of about 2%.
A perspective view schematically showing magnetic domain structure within an amorphous wire constituting such a conventional magneto-sensitive wire is shown in FIG. 6. The magneto-sensitive wire 9 is divided into two layers of a surface layer portion 91 and a core portion 92 by a difference in magnetic domain structure. In the surface layer portion 91, spins are oriented in a certain circumferential direction. Therefore, the spins as a whole are closed as circumference and as a result, no magnetic domain wall exists in the surface layer portion 91.
On the other hand, the core portion 92, which lies inside the surface layer portion 91, has a multi-magnetic domain structure and a lot of magnetic domain walls exist in the core portion 92. In addition, magnetic domain walls exist in a boundary between the surface layer portion 91 and the core portion 92, because the direction of respective spins discontinuously varies.
Since, as discussed above, the surface layer portion 91 has a spin structure (alignment) in which spins are oriented in a certain circumferential direction but the core portion 92 has the multi-magnetic domain structure, the conventional magneto-sensitive wire 9 as a whole has a magnetically composite structure. The magnetic domain walls existing in the multi-magnetic domain structure of the core portion 92 and those existing in the boundary between the surface layer portion 91 and the core portion 92 has caused degradation of the hysteresis characteristics of the magneto-sensitive wire 9 or a sensor employing the same.